Stresses to plants may be caused by both biotic and abiotic agents. For example, biotic causes of stress include infection with pathogen, insect feeding, and parasitism by another plant such as mistletoe. Abiotic stresses include, for example, excessive or insufficient available water, temperature extremes, and synthetic chemicals such as herbicides.
Abiotic stress is the primary cause of crop loss worldwide, causing average yield losses more than 50% for major crops (Boyer, J. S. (1982) Science 218:443-448; Bray, E. A. et al. (2000) In Biochemistry and Molecular Biology of Plants, edited by Buchannan, B. B. et al., Amer. Soc. Plant Biol., pp. 1158-1249). Among the various abiotic stresses, drought is the major factor that limits crop productivity worldwide, and exposure of plants to a water-limiting environment during various developmental stages appears to activate various physiological and developmental changes. Although many reviews on molecular mechanisms of abiotic stress responses and genetic regulatory networks of drought stress tolerance have been published (Valliyodan, B., and Nguyen, H. T. (2006) Curr. Opin. Plant Biol. 9:189-195; Wang, W., et al. (2003) Planta 218:1-14; Vinocur, B., and Altman, A. (2005) Curr. Opin. Biotechnol. 16:123-132; Chaves, M. M., and Oliveira, M. M. (2004) J. Exp. Bot. 55:2365-2384; Shinozaki, K., et al. (2003) Curr. Opin. Plant Biol. 6:410-417; Yamaguchi-Shinozaki, K., and Shinozaki, K. (2005) Trends Plant Sci. 10:88-94), it is also a major challenge in biology to understand the basic biochemical and molecular mechanisms for drought stress perception, transduction and tolerance.
Earlier work on molecular aspects of abiotic stress responses was accomplished by differential and/or subtractive analysis (Bray, E. A. (1993) Plant Physiol. 103:1035-1040; Shinozaki, K., and Yamaguchi-Shinozaki, K. (1997) Plant Physiol. 115:327-334; Zhu, J.-K. et al. (1997) Crit. Rev. Plant Sci. 16:253-277; Thomashow, M. F. (1999) Annu. Rev. Plant Physiol. Plant Mol. Biol. 50:571-599); and other methods which include selection of candidate genes and analysis of expression of such a gene or its active product under stresses, or by functional complementation in a stressor system that is well defined (Xiong, L. and Zhu, J.-K. (2001) Physiologia Plantarum 112:152-166). Additionally, forward and reverse genetic studies involving the identification and isolation of mutations in regulatory genes have been used to provide evidence for observed changes in gene expression under stress (Xiong, L. and Zhu, J.-K. (2001) Physiologia Plantarum 112:152-166).
Activation tagging can be utilized to identify genes with the ability to affect a trait, and this approach has been used in Arabidopsis thaliana (the model plant species) (Weigel, D., et al. (2000) Plant Physiol. 122:1003-1013). Insertions of transcriptional enhancer elements can dominantly activate and/or elevate the expression of nearby endogenous genes, so this method can be used to select genes involved in agronomically important phenotypes, including abiotic stress tolerance such as improved drought tolerance.